本文章的目的是阐明在UIImageView中如何正确的拉伸图片。

UIViewContentMode定义

typedef NS_ENUM(NSInteger, UIViewContentMode) {
    //The option to scale the content to fit the size of itself by changing the aspect ratio of the content if necessary.
    UIViewContentModeScaleToFill,

    //The option to scale the content to fit the size of the view by maintaining the aspect ratio. Any remaining area of the view’s bounds is transparent.
    UIViewContentModeScaleAspectFit, 

    //The option to scale the content to fill the size of the view. Some portion of the content may be clipped to fill the view’s bounds.
    UIViewContentModeScaleAspectFill,     // contents scaled to fill with fixed aspect. some portion of content may be clipped.

    //The option to redisplay the view when the bounds change by invoking the setNeedsDisplay method.
    UIViewContentModeRedraw,              // redraw on bounds change (calls -setNeedsDisplay)
    UIViewContentModeCenter,              // contents remain same size. positioned adjusted.
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,
    UIViewContentModeRight,
    UIViewContentModeTopLeft,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,
};

需要说明的是但凡包含Scale这个单词的值, 都会对原有的图片进行缩放。
这里我们着重说明前三个值

UIViewContentModeScaleToFill

铺满整个画布，图片会变形

UIViewContentModeScaleToFill

UIViewContentModeScaleAspectFit 效果

图片不变形，画布会留有空白

UIViewContentModeScaleAspectFit

UIViewContentModeScaleAspectFill 效果

图片不变形，铺满整个画布，图片会被裁剪

UIViewContentModeScaleAspectFill

另外UIView中的contentMode对应的是CALayer中的contentGravity，那么二者之间的对应关系是什么呢

    UIViewContentModeScaleToFill,          => kCAGravityResizeAspectFill
    UIViewContentModeScaleAspectFit,       => kCAGravityResizeAspect
    UIViewContentModeScaleAspectFill,      => kCAGravityResize
    UIViewContentModeRedraw,               => ???
    UIViewContentModeCenter,               => kCAGravityCenter
    UIViewContentModeTop,                  => kCAGravityTop
    UIViewContentModeBottom,               => kCAGravityBottom
    UIViewContentModeLeft,                 => kCAGravityLeft
    UIViewContentModeRight,                => kCAGravityRight
    UIViewContentModeTopLeft,              => kCAGravityTopLeft
    UIViewContentModeTopRight,             => kCAGravityTopRight
    UIViewContentModeBottomLeft,           => kCAGravityBottomLeft
    UIViewContentModeBottomRight,          => kCAGravityBottomRight
};